1. Field of the Invention
The inventions disclosed and taught herein relate generally to solenoids; and more specifically relate to solenoids used in process control valves.
2. Description of the Related Art
U.S. Pat. No. 3,577,040 discloses an “electronic circuit for actuating a solenoid load from an AC power source in a two-step sequence wherein a high DC voltage is initially applied to “pull-in” the solenoid armature and a lower voltage maintains the armature in a “hold” condition. Silicon-controlled rectifiers (SCR's) provide an electronic switching and rectification of the voltage for operating power. The circuit is controlled by an electrical signal which conditions the SCR's to provide operating voltage during alternate half cycles of the power source and a time delay circuit allows conduction of the “pull-in” SCR for only a few cycles of the power source.”
U.S. Pat. No. 3,660,730 discloses a “circuit for initially applying an unusually large drive voltage to a solenoid coil and for subsequently reducing the applied voltage during the travel of the solenoid plunger. The solenoid coil is serially connected to a first transistor circuit operating as an on-off switch and also is serially connected to a second transistor circuit operating to variably control the voltage applied to the solenoid. A capacitor-charge timing circuit controls the variable transistor and thereby gradually reduces the voltage applied to the solenoid.”
U.S. Pat. No. 7,073,524 discloses a “fail-safe apparatus for controlling fluid flow through a series arrangement of first and second solenoid-controlled valves is provided. The fail-safe apparatus includes fail-safe circuitry for controlling the operation of the first and second solenoid-controlled valves between unactuated and actuated states. Based on a given duty cycle, the fail-safe circuitry selects, actuates, deactuates, and/or maintains in the actuated or unactuated state one or both of the first and second solenoid-controlled valves. To facilitate such control, the fail-safe circuitry may include a switch operable to couple an input voltage across the first solenoid-controlled valve to cause a first current to flow therein. The fail-safe circuitry may also include an energy-transfer device coupled between the first and second solenoid-controlled valves. Depending of the duty cycle, the energy-transfer device is operable to store a potential therein and/or use the stored potential to assist in controlling the first and second solenoid-controlled valves.”
U.S. Patent Application Publication No. 20110094589 discloses a “method of solenoid valve control includes measuring voltage across the solenoid valve and current through the solenoid valve, and using the results to aid in controlling the solenoid valve. For instance, one or both of the measured values may be used to determine when actual engagement of the solenoid valve occurs. An initial lower voltage and lower current can be used, and then as conditions change, the changes in condition can be accounted for by increasing voltage and current to maintain the desired response time of the solenoid valve. By measuring and controlling voltage and current less of a margin can be used, both in setting voltage/current levels and in selecting the time over which a pull voltage/current is utilized. This reduces the wasted energy in the system, as well as reducing the temperature rise in the solenoid valve.”
Patent No. WO2011053392A1 discloses a “method of controlling a solenoid valve (12) includes the steps of: initiating engagement of the solenoid valve by applying to the solenoid valve either a pull-in voltage or a pull-in current; during the applying, monitoring at least one of average voltage across the solenoid valve (40) or current through the solenoid valve (50); from the monitoring, determining completion of engagement of the solenoid valve; and after the determining, reducing either the pull-in voltage to a hold voltage, or the pull-in current to a hold current.”
The inventions disclosed and taught herein are directed to an improved system and method for assuring drop out of a solenoid valve.